A power converter is a power processing circuit that converts an input power waveform, sometimes with, sometimes without, a DC component, to a DC output. A classic AC to DC converter comprises a transformer with a primary side and a secondary side with a rectifying circuit of varying complexity on the secondary side. In the past, most such rectifying circuits used diodes, either singly or in combination, on the secondary side. A more complicated combination that was frequently used in prior art was a half wave or a full wave bridge rectification circuit. Following the rectification part of the circuit was a circuit portion devoted to filtering out the residual AC components, thereby smoothing and regulating the DC output. A DC to DC converter adds a switching function to the primary side of the circuit to enable the transformer to step the output voltage up or down.
The advent of more modern solid state devices made it possible to increase the sophistication of conversion and rectification circuits. In particular, the development of certain types of metal oxide semiconductor field effect transistors (MOSFETs) enabled the development of synchronous rectifying circuits, namely those that use the gating function of a MOSFET instead of a diode to turn current on and off in portions of a secondary circuit. In particular, enhancement-type MOSFETs act as very effective rectifiers when the gate is zero- or reverse-biased during the portion of the cycle in which the secondary current would flow in the reverse direction, but allow current flow when the gate is forward biased.
Self-synchronized rectifiers are MOSFET rectifiers having gate terminals that are driven directly or indirectly by the voltage from a winding of the power transformer in order to provide the rectification of the output of the transformer. Typically, synchronous rectifiers have been used in converters with low output voltages, less than 5 volts, as the gate drive voltages needed to drive the gates of the MOSFET synchronous rectifiers are easily obtainable from the output of the power transformer. This method of driving synchronous rectifiers, shown in FIG. 1, is widely described in the prior art. FIG. 2 shows voltages at relevant points in such a prior art circuit.
As higher converter efficiencies are constantly sought, it has been desired to provide the benefits of synchronous rectification in converters with either higher output voltages or wider input voltage ranges. A problem therefore arises as to how to effectively drive the gates of the MOSFET when the voltages provided by the output of the transformer on higher output voltage converters typically exceed the maximum MOSFET gate drive specification. Likewise, in a power converter that has a wide input voltage range, the voltage applied to the primary of the transformer will have a large range. As a result, the voltage at the output of the secondary of the transformer will also have a large range. Since most MOSFETs have a limitation that the peak voltage at the gate must be typically 20 volts or less, in such circuits the output side voltage will be too large to drive the gates of MOSFET synchronous rectifiers.
Inventors have tried various solutions to these and other problems with prior art MOSFET rectifying circuits. For example, Meyer, et al., Published U.S. Patent Application No. 2004/0240243 A1, published Dec. 2, 2004 describes a method for predicting the proper MOSFET gate drive timing by sensing the diode drop across the synchronous MOSFET and by utilizing a complex algorithm to compute the optimum gate drive timing. This approach incorporates an integrated circuit (IC) component to the design along with its associated cost and complexity. Meyer does not disclose self-driven synchronous MOSFET gate timing.
Other inventors have devised methods for dealing with the voltage-range limitations of prior art self-synchronized drivers and rectifiers. A good example is Bowman, et al., U.S. Reissue Patent No. RE37,5 10 E, reissue date Jan. 15, 2002 (reissue of U.S. Pat. No. 5,590,032, issued Dec. 36, 1996). The Bowman circuit, however, has the drawback of requiring an extra winding on the transformer. On converters that utilize wound magnetics (transformers whose windings are wound using magnet wire) the addition of an extra winding is not too severe an imposition. However, most of today's high-performance DC-DC converters utilize planar magnetics (magnetics that use printed circuit board (PCB) traces as windings).
With planar magnetics, the addition of more windings usually means the addition of another layer to the printed circuit board. Since the board typically is the largest cost component of a DC-DC converter, reducing the number of layers and complexity of the board reduces the cost of the board and thus the cost of the converter.
Even if a planar design is achieved that does not require an extra layer to the circuit board, such a design will still cut into the amount of space available for the power winding, and reduce the copper available to the power winding. Since most of today's DC-DC converters operate at high output currents, cutting into the amount of copper available to the power winding reduces the converter efficiency.
Mao, et al., Published U.S. patent application Ser. No. 2002/0110005 A1, published Aug. 15, 2002, describes a circuit similar to that of Bowman, et al. The Mao, et al. circuit is an effective means of driving the gates of MOSFETs at higher voltages, but it too requires additional windings (in this case two additional windings) on the transformer.
Diallo, et al, U.S. Pat. No. 6,707,650 B2 issued Mar. 16, 2004, demonstrates most clearly the need for the current invention. In the Diallo circuit, as the output voltages of the transformer get larger, the gate drive voltages to the MOSFET gets larger until it exceeds the maximum level. Diallo, et al. attempt to solve this problem by sizing the capacitor (Items 13 and 16 in FIG. 2A of the '650 patent) in series with the gate of the MOSFET such that it forms a capacitive divider with the intrinsic internal capacitance of the synchronous MOSFET. However, as MOSFET gate capacitances are on the order of 1000 to 5000 pF, in order to divide the drive voltage in half, a series capacitor equal to the intrinsic MOSFET gate capacitance would have to be used.
The current state of the art in the power converter industry is to use increased switching frequencies, typically greater than 500 kHz, in order to reduce the size of the magnetics. The trend is also to reduce the MOSFET intrinsic gate capacitance in order to reduce the MOSFET switching times, thereby improving switching efficiency. Unfortunately, the impedance of a 1000 to 5000 pF capacitor at the frequencies (>350 kHz) at which most modern DC to DC converters are run would turn the synchronous MOSFETs on so slowly as such to render them highly inefficient. Therefore, Diallo's method of driving a MOSFET is mostly appropriate on older lower frequency (<100 kHz) converters that utilize large synchronous MOSFTETs (with correspondingly large intrinsic capacitance), so that the series gate capacitor may also be large.
It is an object of this invention to provide a simple, cost-effective, self-driven converter circuit for driving the gates of synchronous MOSFETs that is simpler and cheaper to implement in today's high density, planar magnetic, DC-DC converters. It is a further object of this invention to provide a self-synchronized synchronous drive circuit capable of producing high voltage outputs. It is yet a further object of this invention to provide such a circuit in which MOSFET gate timing is achieved by sensing the secondary of the DC-DC converter's output transformer. It is a further object of the invention to provide such a circuit in which MOSFET gate timing is achieved by sensing the secondary of the DC-DC converters output transformer without including a complex integrated circuit. It is yet a further object of this invention to provide a circuit that can be utilized at virtually any switching frequency and with any output voltage from the secondary of the power transformer.